The present invention relates to an alarm sensor and more particularly, although not exclusively, to an alarm sensor suitable for use in a microwave frequency vehicle intruder alarm.
Another aspect of the present invention relates to an antenna arrangement for use in a vehicle intruder alarm.
Vehicle intruder alarms which rely on detection of the Doppler shift of a reflected microwave signal to indicate movement within a space have been available for several years. One such alarm is described in United Kingdom Patent Application No. GB-A-2280558, the contents of which are incorporated herein by reference.
A problem has been identified with such alarms in that microwave energy leaked from microwave ovens can cause them to be falsely triggered.
Microwave ovens typically employ a cavity magnetron source, the frequency of which will vary as the load Voltage standing wave ratio or VSWR within the oven changes due to rotation of the turntable or mode-stirrer paddle. As microwave ovens typically operate at 2.5 GHz, microwave alarm sensors operating in the 2.45 GHz ISM band can often be subjected to strong interference, traversing their operating frequency.
Microwave ovens also tend to have poorly filtered and regulated power supplies. This typically results in wideband am and fm signals extending several hundred MHZ either side of the centre frequency. These sidebands contain the fundamental mains frequency and many higher order harmonic frequencies.
These modulation phenomena can cause false triggering of low power microwave alarm sensors in two ways. Firstly, the sensor can mistake rectified am sidebands as a Doppler shifted reflection, i.e. as a moving target. Secondly, especially in single antenna systems such as that described in United Kingdom Patent Application No. GB-A-2280558, an oscillator of the sensor can injection lock to the frequency of the powerful interfering signal. In this instance, the sensor will be triggered as the oscillator becomes locked or as it unlocks itself.